The primary task of a power transformer is to act as an electric “gear box” and sometimes to create a galvanic isolation, allowing electric energy to flow from one electrical system to another. The electrical systems interconnected with a transformer usually have different voltages but always the same frequency. The power transformer, in its simplest form, comprises generally at least two windings, a primary winding and a secondary winding. The transformation ratio is defined by the winding turns in the primary and secondary winding and the connection of the windings, e.g., In “delta” or “Y”-connection.
In the transferring of large powers at high voltages over large distances, the geomagnetic field at changes thereof imposes an often quite large quasi-direct current, (DC) in the power line(-s), so called zero sequence current, which direct current accompanies the alternating current phase (AC-phase). The phase lines can be regarded as one line over long distances as the distance between each line becomes relatively small, which causes the induction of the DC current, the zero sequence current, to be equal in all phases, when the geomagnetic field is subjected to changes.
The direct current gives rise to unilateral magnetization levels of any transformer in the system, which may cause the core of the transformer to enter magnetic saturation. This leads to the transformer consuming high magnetizing currents, thus being disconnected, normally by means of a protecting system, which releases the transformer from the system. When a transformer is disconnected, released, from the system, this will of course lead to disturbances In the transmission and distribution of electrical energy.